This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-078312, filed Mar. 21, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a method for injection molding, and more particularly to a method for melting a molding material and storing a constant amount of molten material in a barrel of an injection unit.
An injection unit of an injection molding machine in general has the barrel containing therein a screw for melting a constant amount of molding material and injecting the melted material to a mold. Generally, the molding material is resin formed as pellets, which are stored in a hopper connected to a rear end portion of the barrel. When the screw is rotated in a forward direction within the barrel, the resin is introduced into the barrel from the hopper. The introduced resin is heated and melted. It is supplied to a front end portion of the screw while it is being kneaded, and then successively stored in a top end portion of the barrel. Accordingly, the screw is retreated by the pressure of the molten resin in the top end portion, and the molten resin of an amount substantially corresponding to the retreated distance is stored in the top end portion of the barrel (this step is called a charging step). Then, the molten resin is injected into the mold by moving the screw forward (this step is called an injection step). After the resin is hardened in the mold, the mold is opened and a molded product is taken out.
The amount of the molten resin finally injected into the mold is determined by the position of the screw when the charging step is completed (retreat completion position). Further, the amount of the molten resin varies depending on conditions, such as the temperature in the barrel and the number of revolutions of the screw at a time of introducing the resin in the barrel, and the pressure in the barrel at that time (the back pressure of the screw).
Therefore, to keep the amount of molten resin injected into the mold constant, the temperature in the barrel, the number of revolutions of the screw and the back pressure of the screw are regulated. However, of these conditions, the retreat completion position of the screw is generally fixed for each kind of the molded product.
After the resin in the barrel is completely melted, the properties and the amount of the molten resin injected into the mold are influenced by not only the above conditions but also the manufacturing lot of material resin, the mixture ratio of a recycled material to the material resin, the condition of the screw surface, the condition of internal surface of the barrel and the driving conditions of the screw at the time of injecting the molten resin into the mold. However, since the retreat completion position of the screw is fixed as described above, the amount of the molten resin injected into the mold is varied.
As a conventional technique, a method is known in which the screw retreat speed is controlled to be constant, so that the amount of the molten resin finally injected into the mold may be less varied. However, in this method, since the number of revolutions of the screw is changed to keep the screw retreat speed constant, the back pressure of the screw is varied. Since the number of revolutions of the screw and the back pressure of the screw greatly influence the properties of the molten resin, this method influences the quality of the molded product and therefore is not considered to be preferable. In particular, in the case of injection molding of polyvinyl chloride, decomposition of resin is greatly influenced by the number of revolutions of the screw. Therefore, change in number of revolutions of the screw may cause a serious problem in quality of the molded product.
The present invention was made in consideration of the above problems in the conventional method for controlling the amount of resin introduced and melted in the barrel in an injection molding machine. An object of the present invention is to provide a method for injection molding which can keep the amount and properties of the molten resin finally injected in a mold.
According to the present invention, there is provided a method for injection molding for introducing and melting molding material in a barrel containing a screw, and thereafter injecting the molten material into a mold, the method comprising the steps of:
carrying out pilot production to collect data showing a relationship between a screw retreat distance from a forward limit and screw driving torque when introducing and melting the molding material in the barrel, and integrating the screw driving torque with respect to the screw retreat distance from the forward limit to a retreat completion position of the screw, thereby obtaining a first integral value;
storing as a reference value the first integral value obtained when a conforming item is produced in the pilot production;
carrying out actual production to measure screw driving torque when introducing and melting the molding material, and integrating in real time the screw driving torque with respect to the screw retreat distance from the forward limit to a screw position at each time, thereby obtaining a second integral value; and
monitoring the second integral value, when the second integral value coincides with the reference value, determining that the step of introducing the molding material is completed, and stopping a rotation of the screw.
The present inventors discovered the following matters through their researches. When a molding material is melted in a barrel while it is being introduced therein (i.e., in a charging step), if the temperature in the barrel, the number of revolutions of the screw, the back pressure of the screw, the screw geometry, and the other conditions are constant, the rate of increase in amount of the molten resin stored in a top end portion of the barrel is proportional to the screw driving torque at that time. The present invention is based on this discovery. FIG. 1 shows the relationship between the screw driving torque and the amount of the molten resin stored in a unit time in the top end portion of the barrel.
Owing to the above relationship, the amount of the molding material stored in the barrel can be constant at the charging step completion time, if the value (integral value) obtained by integrating the screw driving torque with respect to the retreat distance of the screw is monitored and the retreat complete position of the screw is controlled using the integral value. As a result, the amount of the molding material finally injected into the mold from the barrel is kept constant.
With the method for injection molding of the present invention, it is unnecessary to change the set values of the number of revolutions of the screw and the back pressure of the screw as control parameters. Therefore, if the temperature in the barrel is set to be constant, the properties of the molding material (i.e., the molten resin) finally injected into the mold from the barrel can be kept constant.
In the case of an electric injection molding machine, the screw driving torque is proportional to the current value of an electric motor. In the case of a hydraulic injection molding machine, it is proportional to the driving hydraulic pressure of a hydraulic motor. Therefore, in the above method, the measurement value of the current or the driving hydraulic pressure can be used instead of the screw driving torque.
Immediately after the screw starts retreating, the relationship between the screw driving torque and the rate of increase in amount of the molten resin stored in the top end portion of the barrel is not necessarily stable. Therefore, it is preferable that the calculation of the aforementioned integral value be started not from the forward limit of the screw but from a reference position preset near the forward limit where the screw is retreated from the forward limit by a predetermined distance.
Preferably, when the molding material introduced and melted in the barrel is injected into the mold, the position where the screw advance speed is changed is shifted at each shot with reference to the retreat completion position in the step of introducing the molding material.
In the above method for injection molding, the screw driving torque may be integrated with respect to the elapsed time from the beginning of the retreat of the screw instead of the retreat distance of the screw.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.